16. tank foundation

7/28/2019 16. Tank Foundation

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A.P.I. 650 and 653 only give general guide lines to

the type of foundation

Number of different types, evaluated on the basis for:

•Risk

•

Cost

•Constructability

•Schedule



Grass, moss growing out from under the tank

What to look for around tank to show Fault

Foundation



Tree’s growing from under the tank



Tank floor laid on river stone



Tank shell has edge settlement



Tank is below ground



Water lying around ring wall foundation



Ground outside foundation cracked



Water lying around tank



Timber railway ties laid under shell holding moisture



Concrete foundation has large cracks



Tank floor annular plate is starting to delaminate



Tank shell is flattening out at the top of the tank



Floating roof jams on the way up



Dike wall out of level



Tank is not level



Tank floor annular is hidden



Insulation holding moisture



Tar laid on top of annular hiding corrosion



Bottom shell course bulges out



Concrete ring wall slopes under tank

Foundation Failure Causing Underside

Corrosion



Poorly constructed or inadequate foundation material



Moisture held in the foundations



Welder sweepings under last floor sheet



Welding of the tank floor not to any code



Steel or wood leveling or layout pins

C t Ri W ll d G t



If the steel tank floor is in contact with concrete,

the moisture in the concrete and the galvanicaction between the dissimilar metals will, over a

period of time, cause the steel to corrode at an

accelerated rate.

Concrete Ring Walls and Grout

Double Bottom Method



Double Bottom Method



Developed in the 1980’s as the only viable

secondary containment method, has failed

in a number of cases



With the old steel floor and a new steel floor

spaced apart with sand or concrete, the oxygenand moisture makes a perfect battery cell and the

new floor becomes the sacrificial anode.



The new steel floor corrodes out at a rate 4 times

faster than the old floor.



C.P.must be designed and installed by experienced

contractors or it may result in increasing the rate of

corrosion of the tank floor.

Any change in the amount of moisture in the

foundation material will cause the resistivity of

the material to change and this may result in theC.P. breaking down.

In some cases where oil impregnated fiber board or



In some cases, where oil impregnated fiber board or

oil sand has been used, nature has done its work

and eaten the oil out of the sand. This can reduce theresistance of the sand and this condition has proven

to accelerate corrosion.



NDT inspection has limitations and flaws and

cannot inspect 100% of the tank floor. It cannot

inspect the critical zone around the shell to

annular weld, lap joints, under the roof

columns, around tank sumps. Sometimespacked rust shows up as good metal.



We have identified that many of theproblems with aboveground storage tanks

start at the foundations. Tank’s not properly

founded are bound to fail.

Tank Foundation



Tank Foundation



When built on good foundations tanks have

lasted 50 and more years



EEMUA Publication No 159



EEMUA Publication No. 159

2.2.2 Problems Caused by Soil Settlement

Even soil settlements are generally not dangerous

because they occur gradually and increaseuniformly. When they become excessive however,

some remedial action may be necessary. The extent

of settlement should therefore be checked, to avoidproblems during operation of the tank.

Pipe connection. Excessive settlement may cause

problems for the pipe connections at the tank shell.

Alternatively the pipe support may settle faster than

the tank on its foundation. Any difference in the level

may seriously overstress the pipe connection.





2.2.3 Additional Problems caused by Uneven SoilSettlements

Uneven soil settlement under the shell of the tank placesthe bottom periphery at greater risk than even soil

settlement. The weak flat bottom plates will follow the

uneven pattern of the soil settlement. The tank will try toform a bridge over the area where local increased

settlement occurs. The bottom plates at that location may

not be properly supported by the tank foundations, but

may be suspended from the shell.

This is an unfavourable condition for the shell to bottom

connection and ultimately a rupture at the inner filletweld may occur.

Tank Settlement into the Foundation EEMUA 159



TankShell

Tank Bottom

Location of Possible

Corrosion

Water and Debris

Collected

Shape afterRepair

Tank Settlement into the Foundation EEMUA 159

Difference in Settlement between Center and



Tank Shell

Tank Bottom

Difference in Settlement between Center and

Periphery

Settlement at center 30% greater than at Shell

EEMUA 159

Shell settlement

Local uneven Settlement under the Tank



Tank

Shell

Tank Bottom

Local uneven Settlement under the Tank

Shell

Possible

Fracture

EEMUA

159

Local uneven settlement under the tank should always be

treated seriously




a) Edge settlement

- Edge settlement may be caused by a tank pad shoulder

with insufficient width or poor quality construction.

- Edge settlement may be caused when the tank pad

shoulder is damaged or eroded by rain and wind

- Edge settlement may occur when the shell

penetrates into the foundation due to poorcompaction of the tank pad under the tank shell





2.2.5 Leakage

If external inspection reveals leakage from the tank

bottom, it is an indication that product is penetratinginto the foundation. This may cause a serious

condition for the safety of the tank and at the same

time cause soil pollution.

Leakage close to the periphery of the shell should be

considered as a major problem requiring urgentattention. It should therefore be remedied

immediately as the leak may cause a local wash out of

the tank foundation under the shell





2.2.5 Leakage

Wash out will cause loss of support for the bottom plates

and may result in a rupture at the inner fillet weld of thebottom-to-shell connection or the bottom lapwelds. This

would cause a sudden complete discharge of the tank

contents.

As it is impossible to predict the probability of a washout

occurring, corrective action should always be taken as

soon as possible

The possibility of a rupture increases when the tank is

filled to a high level





2.2.7 Relevelling of Tanks and Foundations

When settlements have reached their maximum limits,

relevelling of the foundations and the tank will benecessary.

The tank is jacked up to a level 8 feet (2.4 meters)above the foundation to provide sufficient space under

the tank for the repair of the tank foundations

Jacking of tanks has been successfully carried out for

tanks up to 300 foot (90 meters) diameter. The jacking

of large tanks requires a contractor with specialised

experience




2.3 Corrosion of Tanks

General underside corrosion should be slight where

there is a well prepared and maintained foundation.

The major causes of underside corrosion are:

- Poor drainage around tank

- Mill scale, which when present on the underside of the

tank bottom, causes preferential attack

- Water collecting around the tank and migrating under

the tank




2.3 Corrosion of Tanks

- Significant tank settlement that lowers the tank below

a good drainage level. This may lead to corrosion

of the annular plate around the tank perimeter.

- High natural water or underground springs

- Coral or beach sand with high chlorides used for thefoundation.

- Acidic coal based cinders or slags being included in the

foundation.

- Sharp or large stones being included in the foundation.

This may give rise to deep pitting by oxygen

concentrationcell corrosion.




2.3 Corrosion of Tanks- Poor quality of bitumen sand mix. B.S. 2654

- Cathode protection may have been installed after thetank has been commissioned. In this case it

makes it difficult to ensure that sufficient

protective current reaches all areas of the bottom.

- In tanks storing heated products, there are large

differences in the tank to soil potential across the

bottom. This can cause deep random pitting.

Potentials become more anodic from the center of

the tank outwards.




2.3 Corrosion of Tanks- Hot tanks increase water permeation under bottoms and

corrosion rates increase at higher temperatures.

- Stray D.C. currents may cause extremely high local

corrosion. This occurs when an external source of

direct current such as an electric railway causes

current to flow through the ground to the tankbottom.

- Other isolated cases have included rapid galvanic

corrosion resulting from installation of a newbottom on top of an old bottom and higher than

average corrosion rates caused by elevated

temperature.



Corrosion Triangle

Steel

Moisture Air

If all 3 elements are present then corrosion can occur



Corrosion Triangle

Steel

Air


Remove moisture = no corrosion



Corrosion Triangle

Steel

Moisture


Remove air = No corrosion




2.4 Tank Bottoms

2.4.3 Bottom Settlement

2.4.1 Shape of Tank Bottoms due to settlement

2.4.2 Ripples in the tank bottom

a) Edge settlement

- Rain water collected in the depression around the

shell will decrease the quality of the foundation

under the shell and increase the possibility of

edge settlement.




b) Bulges or Depressions in Bottom Plates

- Sometimes it is proposed that these voids are filled with

liquid concrete to give support to the bottom plates

at that location. How ever this is incorrect. Slurry

will not fill these voids but will flow to those

locations where the bottom can move easily, so thatultimately the tank bottom will rest on a number

concrete points instead of being supported evenly

over its entire surface.

- When bulges are formed and not filled with foundation

material (void) there is a risk of rippling when the

tank is filled and the product contained will press theplate downward.

Typical Tank Settlement

Curve



Curve

Settlements cause ovality of



tank shells

The EEMUA



The EEMUA

No. 183

Publication




2. -Tank Foundations: Design

3. -Tank Bottoms: Design

4. -Main Causes of Tank Bottom Leakage

5. -Inspection Records and Techniques

1. -Introduction and Scope




8. -Condition Monitoring and Maintenance

9. -Main Conclusions and Recommendations

7. -Available Systems for Detecting leakage andMinimizing Soil Pollution.

6. -Other measures for preserving the Integrity of Tank

Bottoms.

EEMUA 1832: Tank Foundations



2.6 Sand Pad Foundation with Crushed Stone “Piles”

Stone piles will increase the risk of ground water

pollution from leaking tanks by providing good

leakage paths deep into the soil. Such designs are

not therefore recommended for oil/chemical

storage tanks

2.7 Provision of Oiled Sand or Permeable Bitumen Sand

layer

The provision of an oiled sand or permeable bitumen-

sand layer is considered a very important

requirement for inhibiting underside corrosion. (see

Appendix 1, Fig. 2)

3 Tank Bottoms, Designs EEMUA 183



, g

3.2 Tank Bottoms with Annular Plates

3.4 Double Bottoms

3.3 Tank Bottoms without Annular Plates

3.4 Double Bottoms Fig.10 EEMUA 183



3.4 Double Bottoms EEMUA 183



Fig. 10 The “Letter Box” Method

When the new primary bottom is installed, support for

the bottom has to be re-established both outside andinside the tank shell such that the old bottom and

intermediate shell are effectively redundant.

The letter box method of installing a new bottom in a

single bottom tank has been in use for more than 35

years.

3.4 Double BottomsEEMUA 183



Although there is no requirement to provide support

for the new bottom outside the shell, (API 650) paragraph

1.4.1 requires proper support of the primary bottomand evaluation of the design to verify that the primary

bottom and shell are not over-stressed under the

specified loading conditions including tank settlement.

Such evaluation is not required where the primary

bottom is uniformly supported on both sides of the shell

and not structurally attached to the secondary bottom

or primary bottom support.

3.4 Double Bottoms EEMUA 183



-The installation of double bottoms should be carefully

evaluated, taking into account the relatively high

cost of the design and of the rectification work

required when tanks undergo progressive

settlement.

-Also the lower or secondary bottom cannot be inspected

even when the tank is out of service and

consequently its integrity is always questionable.

EEMUA 183



3.4 Double Tank Bottoms

• Complicated to Install

• Relocation of Nozzles & Manholes• Sensitive to tank Settlements

• Releveling virtually impossible

• Removal of contaminated material from doublebottom (following leakage) potentially hazardous

New Tank Foundation Construction with Liner



7.2 Flexible Membrane Liners in TankEEMUA 183



Foundation

• No alterations to tank structure

• Installation in combination with (scheduled)inspection/lifting/relevelling

• Future inspection/lifting/relevelling not hampered

• Removal of contaminated soil (following leakage) can beexecuted in open air

2 Foundations of Storage TanksEEMUA 183



2. Foundations of Storage Tanks

• 5 types

- type A: Traditional Sand Pad

- type B: Sand Pad with Crushed Rock Annular Section

- type C: Concrete Ring Beam Foundation- type D: Concrete raft

- type E: Underpiled Concrete raft

Type A: Traditional

Sand PadEEMUA 183



Top View Sand PadFoundation

EEMUA 183



Type B: Sand Pad with CrushedEEMUA

183



Rock Annular Section

Type C: Concrete Ring

Beam Foundation EEMUA 183



Type D and E: Concrete EEMUA 183



Raft Foundation

Soil Improvement on un-equal level



Soil Improvement on equal level



Schematic Geometry



Execution of Soil Improvements



• To what depth: same depth every where?

• Extend of excavation: D + 2S + 2T + 4d (+Dd)

• At un-equal soil excavation depths the excavation does

not have a circular shape

• Excavation shall not exceed ground water level (No soil

improvements possible)

• High quality of used materials is required

Effect of Shoulder WidthEEMUA 183



S<1.0 m S>1.5 m

Foundation Conclusions: EEMUA 183



• Width (S) of Tank Pad Shoulder:

- 1.0 m (3FT) for tanks < 15 m (45ft) diameter

- 1.5 m (5FT) for tanks > 15 m (45ft) diameter

• High Pad down-grades the Stability

- Preferable Height above Grade: 600 mm (24inches)

- Crushed Rock Annular Section increases

Stability• Location of Leak Detection Membrane is Crucial

9. Main Conclusions and Recommendations

EEMUA 183



-Codes and standards covering the design and construction

of storage tanks are based on over 100 years of

experience. There is no need to question the integrityof the tank structure as a as a primary container,

provided it is properly designed and built to

recognised standards.

-Effective prevention of ground and ground water pollution

from storage tanks depends on the design and

construction of a good foundation, a properly

construction tank bottom and regular, effective

inspection.


EEMUA 183



-Tank foundations should have a minimum height above

grade as recommended in 4.2 (24 inches 600mm)

-Tank bottom annular plates should be 3/8 in (8 mm)

minimum thickness and butt welded with backingstrips (see 3.2)

-A course crushed rock tank pad shoulder (annular ring)

under the tank shell is preferred to a sand pad. Fig.

3 (see 2.3)


EEMUA 183



-A concrete raft structure with or without piling, though

more costly to build than other foundation types,

may be justified on the basis of long-termmaintenance costs (see 2.5)

-For storage of hot products, tank bottoms should havecone up configuration. However for small tanks a

cone down floor on a concrete raft might be

acceptable (see 4.2)



Compacted Ground

Tank

Ground levelFoundation

Tank Foundation Settlement



Using the tank to preload the new foundation

API APPENDIX B



“RECOMMENDATIONS FOR DESIGN AND

CONSTRUCTION OF FOUNDATIONS FOR

ABOVEGROUND OIL STORAGE TANKS”



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16. tank foundation

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